Reviewing Solar and batteries for retirement

pianotuna wrote:
Hi skipro3,

So how do you size the wattage needed with no data? Or the amp-hours of battery bank? (Gosh, honey, the batteries are dead and the 50 watt solar panel doesn't seem to be charging them fully)

I agree that panel failure is rare--but it does happen and did to one of our members here.

Sizing is fairly fixed if it's an RV. The very cheapest battery and solar panel are the ones you don't have to buy. What I mean is that the first thing to do is reduce the load. LED lights, 2 speed fans, more efficient fans, etc. A few dollars on energy efficient loads saves a whole lot on battery and solar sizing.
Once the load is reduced as much as reasonable, a decent clamp-on amp meter will tell you what the loads you do have draw. My LED lights draw .1amp. My old incandescent bulbs drew 10amps. Meaning I can run 10 LED lights for the same amount of power it took to run just one.

Once it's determined what the loads add up to, battery size and duration needs to be decided. Do you need 1, 2, perhaps 3 days of overcast or rainy weather where the temps are low enough to need the heater on 50% duty cycle after dark? Do you camp where the sun can even reach solar panels on your roof? Are there roof obstructions that will partly shade some of the panels? Are you able to park to optimally capture solar energy? Lots of variation. For us, I have 2 banks of 2 each 6 volt batteries, each bank 200ah of gross capacity. At 50% discharge, that's 200ah of storage without any charging. I use approx. 35ah on my worst case camp conditions; overcast, below freezing night time temp weather. Meaning I can last 3 days without any charging before I dip below 50% drain on my batteries. In order to recover a day's use of 35ah, I have 250 watts of solar panels that will peak at just a hair under 10amps output. 4 hours of peak sunshine will recover my batteries. 8 hours of partial, mottled sun will do the same. Anything less and eventually I'll have to run the genny. More likely I'll run the genny for an hour or so, with the AC output powering my on board charger that outputs up to 35 amps. A tank of fuel will run my genny for 8 hours. I figure I got at least 2 weeks of marginal sunny weather w/1 hour every other day genny use capacity based on my worst case load use.